Ion traps, including RF ion traps, are well established devices that permit ion storage and ejection of stored ions into mass analyzers such as ion cyclotron resonance (ICR) analysers. Kofel, P.; Allemann, M.; Kellerhals, H. P. & Wanczek, K. P. External Trapped Ion Source for Ion Cyclotron Resonance Spectrometry, International Journal of Mass Spectrometry and Ion Processes, 1989, 87, 237-247 describe a rectangular trap in which all sides are held at the same potential, and the stray field from the ICR magnet produces the trapping action. Additionally the use of an ion accumulation RF-trap in or outside the magnetic field is suggested in that document.
S. Michael, M. Chien, D. Lubman, in Rev. Sci. Instrum., 1992, 63, 4277-4284, in U.S. Pat. No. 5,569,917, and in U.S. Pat. No. 5,763,878 describe the use of a 3D quadrupole ion trap as an accumulator and injector into a TOF mass analyser. However, the limited volume of the ion cloud in the traps of this prior art resulted in significant Coulomb interactions between stored ions that greatly affects parameters of resulting ion beams.
Linear ion traps and curved ion traps allowed an increase in the volume of ion cloud and thus reduced the levels at which space charge effects start to affect performance (normally, the allowed number of ions is increased by an order of magnitude or more). Therefore, they have proved to be more suitable for mass spectrometry as well as for ion injection into mass analyzers. Senko M. W. et. al. J. Am. Soc. Mass Spectrom. 1997, 8, 970-976 summarise the use of a range of different traps for use with FT-ICR spectrometers, and describe the use of an octapole ion guide as an accumulator, followed by a second octapole as an injector, ions being transferred out of the ends of the traps in the direction of the trap axis, rather than orthogonally to it. Franzen, in U.S. Pat. No. 5,763,878 describes a trap comprising parallel straight rods with ion ejection orthogonal to the rods. Makarov et. al. describe a curved multipole rod trap with orthogonal ejection, in U.S. Pat. No. 6,872,938.
However, as the ion cloud is distributed along a substantial length of the trap axis this makes subsequent focusing in this direction problematic. A cooled ion cloud lies at the minimum of the RF quasi-potential, and this centre-line (“axis”) may be curved as in U.S. Pat. No. 6,872,938.
Both the recently introduced orbitrap mass analyser, and multi-reflection time-of-flight analysers, require not only high space charge capacity but also the ability to focus ion clouds in time and in all directions, including axially. A curved ion trap that focuses ions through a tiny entrance slot of the orbitrap mass analyzer is described in U.S. Pat. No. 6,872,938. The focusing is provided by the shape of the curved ion trap itself as well as by using curved focusing and deflection optics that lie between the trap and the Orbitrap mass analyser. The deflection optics (z-lens) also serves to reduce pressure problems by leading the ions on a curved path thus blocking direct line of sight (and ion fly over) between the relatively high pressure storage trap and the target mass analyzer or trap.
Though providing high performance, the resulting construction has a number of disadvantages. Firstly, it is complicated to manufacture, secondly, it requires wide slits (with the widths reducing when focal points are approached) leading to increased requirements on differential pumping, and thirdly the trap suffers from the disadvantage that it has a space charge capacity lower than that of the orbitrap itself.
Furthermore, the lenses between the trap and the mass analyser are curved and complex to manufacture and align. In addition, the mass range of ions that can be accumulated and injected into the mass analyser is limited.